Correct. Each letter (from A thru S without N or O) represents an audio tone ranging from about 300hz for "A" up to about 1500hz for "S". When they transmit them they send them in pairs, in this case AE, then MP, each pair in alphabetical order. For example if he said his selcal was M-P-A-E that would be a different, valid code, but P-M-E-A would not because when transmitted the actual tone pairs would be the same as M-P-A-E.